1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device and a manufacturing method thereof. The present invention particularly relates to an active matrix liquid crystal display device having a color filter on a substrate on which switching elements are formed and a manufacturing method thereof.
2. Description of the Related Art
In recent years, the development of an active matrix liquid crystal display device (AMLCD) using thin film transistors (TFT) is actively developed. An on-chip color filter structure is reported in the Japanese Patent Application Laid-open No. Hei-8-122824 (to be referred to as “Prior Art 1” hereinafter), 9-292633 (to be referred to as “Prior Art 2” hereinafter) and so on. In the structure, a color filter is fabricated on a substrate on which TFTs are formed (to be referred to as “TFT substrate” hereinafter). It enables to minimize the discrepancy of the position of the color filter to the TFT substrate.
FIGS. 1(a) and 1(b) show the unit pixel area of an AMLCD to which the on-chip color filter structure disclosed by Prior Art 1 is adopted. FIG. 1(a) is a plan view and FIG. 1(b) is a cross-sectional view taken along line A-A′ of FIG. 1(a).
A TFT substrate 21 consists of a glass substrate 9, scanning lines 1 formed on the glass substrate 9, for selecting a pixel to which a signal is written, signal lines 2 for supplying a signal voltage and TFTs 3 each for driving a pixel formed at the intersection between a scanning line and a signal line. Among these constituent elements, each TFT 3 is comprised of a gate electrode 12 provided on the glass substrate 9, a gate insulating layer 10 provided to cover the gate electrode 12, a semiconductor layer 24 formed on the gate insulating layer 10, a drain electrode 13, a source electrode 14 and a passivation film 11 provided to cover all the above-stated constituent elements. The scanning line 1 is connected to the gate electrode 12, and the signal line 2 is connected to the drain electrode 13. A color filter 8 and a black matrix 4 are provided on the passivation film 11 and an overcoat layer 19 is further formed to protect the color filter 8 and the black matrix 4. The color filter 8 is formed by coating a pigment dispersion type photosensitive negative resist by spin coating and exposing, developing and sintering the resist. A pixel electrode 7 is provided on the overcoat layer 19 and connected through a contact hole 5 to the source electrode 14 of the TFT. Also, an alignment layer (not shown) for controlling liquid crystal molecules to have an arrangement and an inclination (pre-tilt) suited to the operation mode of liquid crystal is provided on the overcoat layer 19 and the pixel electrode 7. A color filter (CF) substrate 22 has a counter electrode 16 and an alignment layer (not shown) provided on a color filter (CF) glass substrate 15. Further, the TFT substrate 21, the CF substrate 22 and a liquid crystal layer 17 put between the TFT substrates 21 and the CF substrate 22 form a liquid crystal element as a whole. With such an on-chip color filter structure, the color filter and the black matrix are formed on the TFT substrate, thereby making it possible to advantageously reduce the alignment errors of the color filter and the black matrix with respect to pixels caused by misregistration between the TFT substrate 21 and the CF substrate 22.
Here, if the film thickness of the color filter is set at 1.2 μm, that of the gate electrode is set at 0.2 μm, that of the semiconductor layer is set at 0.3 μm, that of the drain electrode is set at 0.2 μm and that of the passivation film is set at 0.3 μm, then the film thickness of the color filter on a contact hole portion is 1.0 μm.
Nevertheless, the negative resist used as a color filter is colored and normally low in sensitivity. Due to this, large exposure is required if the color filter is as thick as 1.0 μm, resulting in the problems that it is difficult to make a device small in size and that productivity is low. Besides, due to the large exposure, residue tends to disadvantageously occur onto the lower elements, i.e., the passivation film and the source electrode after development. Further, while pigments are employed by being dispersed into an acrylic resin, if the sensitivity of this acrylic resin increases, the pigments are hardened only in the vicinity of the surface of the color filter (in a region from 0.3 to 0.5 μm from the surface of the color filter) due to photo-crosslinkage and the shape of the neighborhood of the color filter and that of the contact hole portion are undercut after development. This causes the problems that contact resistance between the pixel electrode and the source electrode increases, adhesion between the color filter and the substrate deteriorates, and the like. That is, in case of the LCD with the on-chip color filter structure according to Prior Art 1, the color filter is thick on the contact hole portion and the neighborhood of the pixels and the photo-crosslinkage of the color filter, therefore, occurs to the surface of the color filter. As a result, this device has disadvantages in that it is difficult to make the device small in size and to improve a aperture ratio as well as productivity is low.
Moreover, with the on-chip color filter structure, in the pigment dispersion type photosensitive negative resist serving as a color filter, a photosensitive acrylic material sensitive to i-rays, g-rays and h-rays is employed as a base resin. As can be seen from the relationship between the film thickness of this high photosensitive color filter resist (e.g., CM-7000 manufactured by Fujifilm Olin Co., Ltd.) and photo-crosslinkability (shown in FIG. 2), the color resist is low in transmittance with respect to i-rays and photo-crosslinkable property suddenly decreases at around 0.3 to 0.5 μm from the surface since the color resist is colored. Normally, the film thickness of the color filter is 0.3 to 0.5 μm. However, if development time is long or over-development occurs for the above-stated reasons, the color filter is isotropically dissolved on the bottom thereof having low photo-crosslinkability and the filter disadvantageously becomes an inverse tapered shape.